Semiconductor wafers are commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that must be smoothed out before generating the next circuit layer.
Chemical mechanical polishing (CMP) techniques are used to polish and planarize the raw wafer and each layer of circuitry added. Available CMP systems, commonly called wafer polishers, often use a rotating wafer carrier head that brings the wafer into contact with a polishing pad rotating in the plane of the wafer surface to be planarized. A chemical polishing agent or slurry containing microabrasives is applied to the polishing pad to polish the wafer. The wafer carrier head then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. The mechanical force for polishing is derived from the rotating table speed and the downward force on the wafer carrier head.
A conventional way to axially align the rotating polishing pad with the wafer carrier is to use a rigid surface on one body, such as a body that supports the polishing pad, and a gimbal on the other. A problem occurs when the polishing surface of the rotating polishing pad is smaller than the wafer surface, and especially as the polishing surface moves off an edge of the wafer during polishing. Since the gimbaled surface tends to tilt at the edge, alignment of the wafer to the polishing surface becomes difficult, and is preferably accomplished by using all rigid surfaces. Many polishing techniques, such as CMP, however, require at least one dynamic self-aligning surface to align the polished layer to a previously processed underlying layer of the wafer.
Thus, there is a need for a method for polishing wafers where the wafer is dynamically aligned to a polishing surface and then rigidly held in place when the alignment is accomplished.